1. Field of the Invention
This invention relates to the field of refrigeration, more particularly the production of a low-temperature gas stream, and it has as its object a heat exchanger device, as well as an NMR installation that uses such a device.
2. Description of the Related Art
In many technical fields, and in particular in the field of Nuclear Magnetic Resonance (NMR), it is necessary to use a low-temperature gas stream, i.e., in particular lower in temperature than 100° K and in particular close to the temperature of liquid nitrogen (77.3° K).
These cold gas streams are used in particular to cool the sample that is to be analyzed and the equipment parts that are in contact with the latter or to keep them at a low temperature, but also, if necessary, to generate mechanical, static or dynamic actions.
Thus, in NMR (MAS) of the solid, the sample tube, called a rotor, is in rapid rotation (at several KHz, and even several tens of KHz) during the measurement phase, whereby the rotation of the sample makes it possible to improve the quality of the measurement spectra.
There is also a great advantage in carrying out MAS (Magic-Angle Spinning) analyses at low or very low temperatures for different scientific reasons (phase transitions, monitoring of reactions, etc.).
The low-temperature MAS analyses require special equipment and very cold gases, preferably, in the current context, as close as possible to the temperature of liquid nitrogen (77.3 K).
In a known manner, the MAS-type NMR analyses are generally carried out by means of a probe that comprises a stator part and a rotor part, constituted by the sample tube that rotates in the stator.
It is possible in particular to note the following points regarding the constitution and the operation of a MAS probe of the above-mentioned type:                Presence of two gas bearings (supplied by a so-called “bearing” gas) that support the sample tube that is called a rotor (gaseous levitation);        The rotation of the rotor is carried out by a so-called “drive” gas whose pressurized ejection at one or several nozzle(s) brings about a more or less rapid rotation of the sample that is placed in the rotor (equipped with blades or analogous formations);        The temperature of the sample is monitored by a third gas designated as “VT.”        
By way of numbered examples that illustrate the characteristics of the gas streams that are currently implemented at the level of an MAS NMR probe, it is possible to indicate:                “Bearing” gas pressure of 2 to 4 bar/flow rate: up to 50 to 80 Nl/minute;        “Drive” gas pressure of 1 to 2 bar/flow rate: 10 to 20 l/minute, typically (depends on the speed of rotation);        “VT” gas pressure that is less than 1 bar/flow rate: 30 l/minute.        
In practice, the gas that is used in these applications is nitrogen for reasons of cost, comfort of use and limitation of risks. However, other types of gas can be considered, such as argon, helium, oxygen, . . . , based on applications and temperatures to be reached.
In a conventional manner, it is possible to produce low-temperature gas streams very simply by making the latter circulate in coils or exchangers that are immersed in a liquid gas bath, advantageously of the same nature as the circulating gas that is to be cooled.
This simple technique, however, has a major drawback, which is very detrimental to the applications mentioned above.
Actually, the cooling of a pressurized gas brings about a condensation of said gas and, consequently, in the coil or pipe mentioned above, the formation of liquid gas droplets. In addition, the flow rate then becomes unstable, and there are pressure fluctuations.
However, in particular in the context mentioned above, it is extremely important to provide a stream of dry cold gas, i.e., droplet-free gas, for the production of the bearings that maintain and guide the rotor in rotation.
Actually, if the so-called “bearing” gas undergoes condensation bringing about the formation of droplets, the result is a major malfunctioning of the bearings (because of the pressure fluctuations that are generated). This malfunctioning and the instabilities that it involves can produce a physical contact between the rotor and the stator that can damage and even destroy the sample tube, with pollution and/or damage of the stator, and even of the probe.
Thus, the primary problem posed in this invention is to propose a device that makes it possible to provide at least one stream of dry cold gas, in particular in the context described above, and to prevent the above-referenced drawbacks.
The publication “Apparatus for Low-Temperature Magic-Angle Spinning NMR (“Appareil pour la RMN basse temperature avec rotation à l'angle magique”), P. J. Allen et al., Journal of Magnetic Resonance 92, 614-617 (1991), proposes a heat exchanger device that can deliver at least one pressurized gas stream that is cooled to an essentially constant temperature. The gas streams circulate, in the device according to this document, in a coil or a corresponding analogous pipe of which a functional part is immersed in a liquid fraction of a second gas that is present in a chamber of a corresponding container, whereby the wall of the container that delimits said chamber is in contact with a liquid fraction of a third gas, so as to constitute a condensation surface for the second gas.
The device according to this publication actually comprises two intertwined coils providing the “bearing” gas and the “drive” gas, whereby these two coils are immersed in a liquid N2 gas bath that is present in a container that is itself immersed in a liquid N2 bath.
However, this publication does not mention the problem of providing dry gas, in particular after the sample tube is changed, and the internal pressure of the container is simply monitored by a pressure regulator. The construction of the chamber in copper can produce a super-cooling of the liquid gas of the container, upon shutdown, and can bring about the condensation of the gas that circulates in the coil.